The present invention relates to a process of prolonging the life of the hydrogenation catalyst used in the catalytic hydrogenation of glycolic acid esters to produce ethylene glycol.
Generally, the hydrogenation of esters to alcohols is well known, as can be seen, for example, from U.S. Pat. No. 1,605,093. According to this patent, a copper catalyst is used in the hydrogenation. Also, esters have been reduced to alcohols by using various means, such as lithium aluminum hydride or sodium plus an alcohol.
U.S. Pat. No. 2,285,448 granted June 9, 1942, discloses the liquid phase hydrogenation of the esters of glycolic acid to produce ethylene glycol. The liquid phase hydrogenation is carried out at pressures in excess of 100 atmospheres and preferably above 400 atmospheres with temperatures between 125.degree. and 325.degree. C. According to this patent, a copper-magnesium catalyst is preferred. It is stated in the patent at page 1, column 2, lines 47-52 that in place of magnesium oxide, other metal oxides may be employed to promote the activity of the copper oxide.
U.S. Pat. No. 2,305,104 is likewise concerned with the hydrogenation of alkyl glycolates. This patent relates to a vapor phase hydrogenation process using apparatus wherein catalyst degradation is reduced to a minimum. The process is carried out by reacting the alkyl ester of glycolic acid and hydrogen in a catalyst charged reaction zone and subsequently, mixing the gases issuing therefrom with sufficient additional hydrogen to maintain the reaction products in the vapor phase and to maintain the esters and hydrogenated products in the vapor phase, and reacting the resulting mixture in another catalyst charged reaction zone.
British Pat. No. 575,380 relates to a process of hydrogenating an ester of glycolic acid to produce ethylene glycol under conditions such as to minimize loss of catalyst activity. The process is carried out in the vapor phase, wherein the gaseous mixture containing hydrogen and between 1.5% and 8% by volume of the ester of glycolic acid is contacted with the hydrogenating catalyst at a pressure between 10 and 75 atmospheres and at a temperature between 150.degree. and 300.degree. C.
Commonly assigned U.S. Pat. Nos. 3,911,003 and 4,087,470 granted, respectively on Oct. 7, 1975 and May 2, 1978, the disclosures of which are herein incorporated by reference, are concerned with the preparation of glycolic acid and diglycolic acid and their reduction to ethylene glycol or diethylene glycol. The process disclosed in U.S. Pat. No. 4,087,470, involves the steps of (1) contacting formaldehyde, carbon monoxide and hydrogen in the presence of hydrogen fluoride to form glycolic and diglycolic acids; (2) contacting the acids of step (1) with ethylene glycol, diethylene glycol or mixtures thereof to produce ethylene glycol glycolate and diglycolate, diethylene glycol glycolate and diglycolate, or mixtures thereof; and (3) contacting the ester glycolate and diglycolate products of step (2) with hydrogen in the presence of a hydrogenation catalyst to produce the desired ethylene glycol or diethylene glycol.
In accordance with the aforesaid U.S. Pat. No. 4,087,470, the esters of step (2) above are prepared by first purifying the acid product of step (1) essentially by removal of the hydrogen fluoride catalyst. Pursuant to U.S. application Ser. No. 931,333, filed Aug. 7, 1978, HF is most effectively removed by heating to a temperature above about 130.degree. C., preferably with gas or vapor stripping, such treatment resulting in the formation of a polymer of a glycolic acid.
Glycolic acid possesses characteristics of both a carboxylic acid and an alcohol and is accordingly capable of forming linear esters by reaction between a hydroxyl group of one molecule of glycolic acid and the carboxyl group of another molecule of glycolic acid, with the simultaneous formation of water. These esters are monoglycolide or polyglycolides and may be formed even in concentrated aqueous solution, as described in pages 632 and 633 of Vol. 10 of the second edition of Kirk-Othmer's "Encyclopedia of Chemical Technology."
Following dehydration, the anhydrous glycolic acid (i.e., the various dehydrated forms of glycolic acid, and in particular a mixture of glycolic acid and polyglycolides) as prepared, for example, in accordance with the aforesaid U.S. Pat. No. 4,087,470 is reacted with an alcohol under esterification reaction conditions to produce the glycolate esters, as shown, for example, in U.S. Pat. No. 2,331,094. Suitable alcohols are methanol, ethanol, n- and iso-propanol, n- and iso-butanol, octanol and higher straight and branched-chain alcohols, polyhydric alcohols, such as ethylene glycol and diethylene glycol, as well as the mixtures or particular fractions of the mixtures of alcohols obtained by the hydroformylation of olefins.